1. Field of the Invention
The present invention relates to coordinate input devices, and particularly, to a coordinate input device which allows desired input by sliding an operating part and which is suitable for, for example, personal computers, mobile phones, and video game units.
2. Description of the Related Art
FIGS. 13 and 14 illustrate a conventional coordinate input device which can be operated by sliding an operating part to perform a desired input operation. Such a conventional coordinate input device is disclosed in Japanese Unexamined Patent Application Publication No. 2003-84916. The coordinate input device includes an upper casing 51 at the top portion of the device, and an insulative lower casing 52 which faces the upper casing 51 and is disposed below the upper casing 51.
The upper casing 51 is provided with a flat electrode section 51a, a shaft guide 51b, and a circular operation hole 51c. Specifically, the electrode section 51a extends around the outer periphery of the upper casing 51, and the shaft guide 51b protrudes from the central portion of the electrode section 51a. The circular operation hole 51c extends through the center of the shaft guide 51b. 
Furthermore, a pair of caulkers 51d extends downward from two corresponding sides of the upper casing 51, i.e. at the left and the right of the drawing.
On the other hand, the lower casing 52 includes a cover 52a around the outer periphery of the lower casing 52, and a cavity 52b surrounded by the cover 52a. The cover 52a of the lower casing 52 is provided with a caulking protrusion 52c for caulking the upper casing 51. The caulking protrusion 52c extends to a predetermined height.
The central portion of a bottom plate 52d forming the cavity 52b is provided with a switch-holding section 52e which extends downward to a predetermined depth.
Furthermore, the bottom plate 52d is provided with a fixed electrode 53 which includes, for example, four equally-divided fan-shaped electrode components, i.e. first to fourth electrode components, in the circumferential direction of the bottom plate 52d. 
The fixed electrode 53 further includes reeds 53a which are connected to the corresponding first to fourth electrode components. The reeds 53a extend toward the exterior of the cover 52a. 
The bottom surface of the switch-holding section 52e of the lower casing 52 is provided with a switch circuit 54. The switch circuit 54 includes a circular central fixed-contact 54a and a peripheral fixed-contact 54b having, for example, a horseshoe shape and disposed distant from the central fixed-contact 54a. 
The central fixed-contact 54a and the peripheral fixed-contact 54b are respectively connected with external terminals 54c and 54d which extend toward the exterior of the bottom plate 52d. 
Furthermore, an operating shaft 55 extends through the shaft guide 51b of the upper casing 51. The operating shaft 55 includes an operating part 55a protruding upward and a flanged part 55b. 
The operating shaft 55 further includes a supporting part 55c below the flanged part 55b and having a larger diameter than the operating part 55a, and a contact-pressing part 55d protruding downward from the center of the bottom surface of the supporting part 55c. 
According to the operating shaft 55, the operating part 55a extends upward through the operation hole 51c of the upper casing 51, and the flanged part 55b is in contact with the inner surface of the shaft guide 51b. Thus, the operating shaft 55 is slidable in any horizontal direction, and moreover, can be pressed vertically.
The cavity 52b contains a toric-shaped movable electrode 56. The central portion of the movable electrode 56 is provided with a supporting hole 56a which engages with the supporting part 55c of the operating shaft 55.
Furthermore, the outer diameter of the movable electrode 56 is smaller than the inner diameter of the cavity 52b, such that a resilient member 57 is disposed in a space between the inner peripheral surface of the cavity 52b and the outer peripheral surface of the movable electrode 56.
The resilient member 57 is an O-shaped ring whose inner diameter is equal to or less than the diameter of the movable electrode 56. The cross section of the resilient member 57 is substantially oval.
Since the resilient member 57 is disposed in the space between the inner peripheral surface of the cavity 52b and the outer peripheral surface of the movable electrode 56, the biasing force of the resilient member 57 allows the movable electrode 56 to be positioned at the center of the cavity 52b. 
Moreover, adjacent to each of the top and bottom surfaces of the movable electrode 56, an insulative sheet 58 is provided.
The switch-holding section 52e of the lower casing 52 contains a metallic contact 59 whose central portion is projected upward.
The metallic contact 59 in the switch-holding section 52e is in contact with the tip of the contact-pressing part 55d of the operating shaft 55, and the periphery of the metallic contact 59 is electrically in contact with the peripheral fixed-contact 54b of the switch circuit 54.
Referring to FIG. 14, the operation of such a conventional coordinate input device will now be described. Firstly, the operating shaft 55 is in the initial state. This initial state refers to a neutral state in which the movable electrode 56 and the operating shaft 55 are positioned at the center of the cavity 52b by the resilient force of the resilient member 57.
In the initial state, a controller (not shown in the drawings) is capable of determining the electrostatic capacitance generated between the electrode section 51a of the upper casing 51 and the electrode components of the fixed electrode 53 in the lower casing 52.
For example, when a horizontal force is applied to the operating shaft 55 in the initial state in a direction indicated by an arrow X, the movable electrode 56 slides toward the left of the drawing such that the left portion of the resilient member 57 is compressed and resiliently deformed.
Thus, the electrostatic capacitance between the fixed electrode 53 and the movable electrode 56 at the left side of the drawing increases from the initial value.
At the same time, the electrostatic capacitance between the fixed electrode 53 and the movable electrode 56 at the right side of the drawing decreases from the initial value.
When the controller detects such a change in electrostatic capacitance, a cursor on a display screen of, for example, a mobile phone (not shown in the drawings) can be moved to a desired position at the left side of the display screen.
When the cursor is shifted to the desired position and the operating shaft 55 is pressed vertically, the contact-pressing part 55d of the operating shaft 55 presses against the metallic contact 59.
Thus, the metallic contact 59 becomes resiliently deformed such that the tip of the deformed metallic contact 59 comes into contact with the central fixed-contact 54a. The metallic contact 59 thus electrically connects the central fixed-contact 54a and the peripheral fixed-contact 54b so as to switch the contact of the switch circuit 54.
For example, the switching of the contact of the switch circuit 54 can allow a message, corresponding to the position of the cursor on the display screen, to be opened.
Furthermore, when the horizontal force or the vertical force applied to the operating shaft 55 is released, the resilient force of the resilient member 57 or the resilient force of the metallic contact 59, respectively, forces the operating shaft 55 to return automatically to the neutral position, i.e. the initial state.
Such a conventional coordinate input device, however, is problematic in that the device is large in size. This is due to the fact that, for allowing the operating shaft 55 to slide by a large distance, the thickness, of the resilient member 57, whose cross section is substantially oval, must be made large so that the resilient member 57 can be deformed in cross section.
Moreover, a large operational force is required for sliding the operating shaft 55 so as to cross-sectionally deform the resilient member 57. Otherwise, the movable electrode 56 cannot be slid properly.
Consequently, this may cause problems for, for example, women and children since they may not be able to slide the operating shaft 55 by a large distance and thus may not be able to perform a desired coordinate input operation.